Silicon and other semiconductor devices are sometimes employed at sufficiently high voltages to precipitate a current avalanche and electrical breakdown of the device. Without adequate current limitations, such breakdown may lead to failure of the device. If nothing else, the breakdown may cause a failure of the circuit in which the device is employed, to perform as required. Thus, designers of semiconductor devices often need to design the device so that it can withstand high voltages without electrical breakdown. For example, it is desirable for the curvature of diffused junctions in the device to have as large a radius as possible, since short-radius junctions are subject to electrical breakdown at lower voltages than high-radius junctions.
Regardless, it remains desirable to provide additional means for deferring electrical breakdown to even higher voltages.
An example of a device often subjected to high voltages is a semiconductor-controlled rectifier (SCR), sometimes referred to as a silicon-controlled rectifier or thyristor. Various diodes, bipolar transistors, MOSFETs and the like may also have reverse voltage limitations which need to be taken into account. An SCR is typically in a circuit having an alternating current so that the gate of the SCR controls the proportion of each forward cycle of the alternating current that is conductive, while resisting all current flow during the reverse half of the cycle. The breakdown voltage of the SCR must be greater than the peak voltage of the alternating current in the circuit in which the SCR is used.
In an SCR, electrical breakdown can initiate adjacent to the gate at a relatively lower voltage or adjacent to the anode at a relatively higher voltage. It is desirable to provide means adjacent to the gate for shaping the depletion region, thereby minimizing electric field and avoiding low-voltage breakdown. The breakdown of the SCR can then be limited by the breakdown voltage adjacent to the anode. It is also desirable, however, to shape the depletion region and electric field adjacent to the anode so that electrical breakdown in that vicinity is also deferred to higher voltages.